1. Field of the Invention
The present invention relates to a technique of notifying a user of degradation in image quality caused by a foreign substance sticking to the surface of, e.g., an optical low-pass filter arranged in front of an image sensor in an image capturing apparatus which uses an image sensor such as a CCD or CMOS sensor.
2. Description of the Related Art
When the lens is detached from the camera body of a lens-interchangeable digital camera, dust particles floating in air may enter the camera body. The camera incorporates various mechanical units such as a shutter mechanism which mechanically operate. When these mechanical units operate, dust such as metal powder may be generated in the camera body.
When a foreign substance such as dust sticks to the surface of an optical element (e.g., optical low-pass filter) arranged in front of an image sensor included in the image capturing unit of the digital camera, the image of the foreign substance is contained in a sensed image, resulting in degradation in the image quality.
A camera using a silver halide film feeds the film in every shooting. Hence, images never contain the same foreign substance at the same position continuously. However, the digital camera requires no operation of feeding the film frame in every shooting, and therefore, sensed images continuously contain the same foreign substance at the same position.
To solve this problem, Japanese Patent Laid-Open No. 2004-222231 discloses a method of correcting a luminance change caused by a foreign substance on the basis of a reference image. In Japanese Patent Laid-Open No. 2004-222231, an object having a uniform luminance is sensed to obtain a reference image, and a transmittance map is generated from the luminance distribution of the image. Appropriate gain correction is performed for an image sensed by a user to correct a change in transmittance caused by a foreign substance, thereby obtaining a high-quality image.
In the conventional method, however, the user cannot confirm the position and size of a dust particle before or during shooting. For this reason, the user cannot predict the degree of influence of the dust image on a sensed main object.